It is well-known that transparencies for use with overhead projectors can be produced by imagewise deposition of liquid ink of various colors onto thin, flexible, transparent polymeric substrates. Such imagewise ink deposition can be performed by such apparatus as pen plotters and ink jet printers of the type normally used for applying images to paper. In the imaging of paper by such ink deposition methods, the porous nature of the paper is an important factor in the drying of the ink, as well as in the final image quality. In contrast to paper, however, the non-porous nature of transparent, polymeric substrates makes depositing of ink from pen plotters or ink jet printers thereon very difficult, with the result that poor quality images of low durability are formed.
A common characteristic of inks used in pen plotters and ink jet printers is that they are exposed to open air, prior to imaging, for long periods of time. Yet, even after long term exposure to open air, the ink must function in a satisfactory manner, without deterioration, particularly without loss of solvent. In order to meet this requirement, ink formulations typically utilize solvents of very low volatility, such as water, ethylene glycol, propylene glycol, and other like solvents. Because of the low volatility of the solvents in these ink formulations, solvent evaporation as a mechanism for drying the ink after imaging is very limited.
In the case of printing on paper by means of ink jet printers, images are composed of small dots, with the spacing between the dots being of such a magnitude as to allow lateral diffusion of the ink on the substrate to just fill areas between adjacent dots. In the case of printing on film by means of ink jet printers, the situation is quite different, with little or no ink spreading, and with a tendency toward irregular beading up of the ink into droplets of various sizes, with the droplet sizes being unrelated to the intended dot size. When such beading occurs, image quality is diminished.
Problems of ink beading and failure to dry arise because most transparent films suitable for graphics applications, in addition to being non-porous, are also, in many cases, hydrophobic. Imageability of transparent, polymeric film with inks containing solvents of low volatility has been improved by applying hydrophilic, water absorbent, ink-receptive layers to transparent, polymeric film substrates. In addition to absorbing the low volatility solvents found in ink formulations, these layers must exhibit ink-receptivity, durability, non-tackiness, and the image quality required of commonly accepted graphic arts materials.
It is desirable that water-absorbing polymeric compositions retain some degree of physical durability, dryness to the touch, and non-tackiness even after absorbing significant amounts of water or aqueous ink, as would occur in the imaging process. Because polymeric materials have a strong tendency to be softened or even dissolved by the absorption of liquids, the absorption of significant quantities of liquids is inconsistent with retention of physical and handling properties desired in graphic arts materials. Attempts at resolving these apparently conflicting goals, while at the same time meeting the additional requirement of retaining transparency, have often involved the use of polymer blends.
In U.S. Pat. No. 4,503,111, coatings comprising blends of polyvinyl pyrrolidone and either polyvinyl alcohol or gelatin are disclosed. The materials disclosed in U.S. Pat. No. 4,503,111 exhibit good ink-absorption when used in ink jet printers, but tend to be tacky in imaged areas, and at high humidity, exhibit tackiness even in non-imaged areas.
Another undesirable feature of the polymer blends disclosed in that patent is dimensional sensitivity to moisture content, resulting in a tendency for the film to curl when subjected to heating or changes in humidity.
Polymeric blends that are less tacky and moisture-sensitive, but which still maintain water absorptivity, are disclosed in U.S. Pat. Nos. 4,300,820 and 4,369,229, wherein blends of hydrophilic polymers with terpolymers having hydrophobic and hydrophilic parts, along with acid functionalities, are disclosed. Although these patents do not disclose ink-receptive coatings, the polymeric blends disclosed therein have been found useful as water absorbent layers capable of retaining a degree of durability and non-tackiness even after absorbing significant quantities of water. However, U.S. Pat. Nos. 4,300,820 and 4,369,229 do not mention either graphic arts materials or the various properties characterizing image quality which are important in such materials.
European Patent Application No. EP 0 233 703, discloses blends of acrylic polymers with polyvinylpyrrolidone (PVP) for use in ink-receptive layers, wherein compatibility between the hydrophilic PVP and the hydrophobic acrylic polymer is achieved by incorporating carboxylic acid groups into the acrylic polymer. It does not appear possible to adjust the formulations disclosed therein to simultaneously provide adequate drying, low tack, and acceptable lateral migration of ink in many of the commercially available ink jet printers.
In addition to pre-imaging optical requirements, such as clarity, polymeric blends used in transparent ink-receptive layers must also exhibit satisfactory post-imaging quality. It is desirable that the ink dry, tack-free, in a reasonably short time. A typical requirement for drying may be that the imaged sheet be sufficiently dry and tack-free to allow stacking in the time required to image the next sheet. Additionally, considerable spreading of the ink dots is desirable to achieve adequate color density. At the same time, any spreading of ink dots should be adequately controlled so as to produce sharp, smooth lines of demarcation between contrasting colors. It has been found that this combination of features is not readily attainable in a single formulation. For this reason, multi-layer coatings have been found useful.
U.S. Pat. Nos. 4,225,652, 4,301,195, and 4,379,804, disclose ink-receptive materials having multiple layers, with the first, or underlayer, being highly ink-absorbent, and the second, or topcoat, layer, being ink-permeable, while at the same time being quite durable, even after the application of ink. Because the topcoat layer need not retain any significant amount of ink, but merely allow ink to pass through to the underlayer, many of the softening effects resulting from high levels of solvent retention can be avoided in the topcoat layer. Further, since the underlayer, which is softened by the absorption of ink, is protected by the topcoat layer, the overall image is more durable and less tacky. It has been found, however, that when highly water-absorbent materials, such as, for example, polyvinyl pyrrolidone, are used for the ink-absorbent underlayer, the degree of softening which occurs is sufficient to cause severe reduction in the durability of the overall imaged layer, and that topcoating is not sufficient to overcome this reduction in durability. When materials which are less susceptible to softening due to ink-absorption are used for the underlayer, the ink-absorbing ability of the system is likely to be unacceptably low.
Another disadvantage of presently available ink-receptive layers is that their high level of sorption capacity leads to poor dimensional stability, which, in turn, leads to curling of the film whenever moisture content changes, either due to imaging, or due simply to changes in humidity or to drying out due to the heat encountered on the stage of the overhead projector.
Curl can be reduced by applying the same image receptive layer to both sides of the film backing, so that any expansion or contraction is applied symmetrically to both sides of the sheet. This adds to manufacturing cost, and still may not completely eliminate curl, especially in cases where moisture content differs on opposite sides of the film.
Other methods of imparting dimensional stability to moisture sensitive materials include chemical cross-linking, as is done in photographic gelatin coatings, and permanent swelling, as occurs when humectants are added to wood, paper, or other fibrous cellulosic materials. It is well-known, however, that chemical cross-linking reduces moisture absorption, and is therefore not a desirable method for stabilizing ink-receptive coatings.
In the case of ink absorbing layers, however, the addition of humectants to provide permanent swelling due to moisture saturation is not an acceptable means for imparting dimensional stability, since such saturation greatly reduces the ability of such a layer to absorb aqueous solvent.
The present invention provides improved materials for use in transparent ink-receptive layers. The improvements disclosed herein result in faster drying, lower tack of imaged sheets, and less curl of the sheets.